1. Technical Field
The present inventions each relate to methods and arrangements for manufacturing spray formed metallic articles; more specifically the inventions relate to such inventive aspects as heat treatment processes for minimizing internal stresses and deflections in produced articles, manipulating temperature and the time periods for hold certain temperatures to establish prescribed multi-phase metallic compositions in produced articles also for minimizing internal stresses and deflections in produced articles, and a unique one dimensional based model utilized for affecting feed-forward control over the spray form process.
2. Background Art
It is a known process to spray form certain articles using moltenizing arc guns having metal feed wire supplied thereto. Further, it is known that volumetric changes occur during cooling of the metal that can produce significant detrimental effects in the finished product, one of the more significant of which is typically manifest as internal stress that is trapped within the substantially rigid article after its manufacture. It is not uncommon for stresses of magnitudes high enough to warp or otherwise cause deformation and deflection in the finished article to occur in uncontrolled spray processes, and even minor deflections due to internalized stress can render conventional spray form processes unusable when precision tooling is required for particular finished products or articles. In another aspect, as the technology and processes for spray forming metallic articles advance, the manufacture of progressively larger monolithic bodies becomes feasible. As a result, however, the volumetric changes experienced during the cooling of the metal in such larger spray formed bodies is becoming more pronounced due, for example, to their greater sizes and thicknesses. The detrimental effects of these volumetric changes experienced within a spray formed article have long been appreciated; not the least of which can be, and often is, the inducement of internal stresses within the article itself.
It is known that molten steel can undergo various phase changes, for example, from austenite to ferrite, pearlite, bainite, martensite, and various combinations thereof as it cools, and that these phase changes involve positive volumetric changes. Previously, it has been postulated that the transformation to martensite can offset the stresses caused by shrinkage that also occurs during cooling. The focus of this idea was that a balance between the positive volumetric changes and the thermal contractions could be effected by the transformation to martensite.
When steel is initially sprayed and still at a high temperature, it is typically one-hundred percent austenite, and as the steel begins to cool, the austenite begins to change into other sister phases. At a relatively high temperature, the first phase transformations are primarily into ferrite and pearlite. As the temperature moves lower, the next transform is into bainite, and at the lowest temperature, transformation to martensite occurs. Even though it was known that these transformations were occurring as the steel cooled, it has been the martensite transformation which has been primarily capitalized upon to provide stress relief to the spray formed body or article.
A current approach to controlling the spray forming process has been through temperature control. In such an approach, temperature is used as an input for robotically manipulating the spray guns. In this approach, the moltenized metal spray is produced using, for example, a number of twin-wire arc plasma torches or guns. The movement and performance of the guns may be automated via computer/robot controls, the surface temperature(s) of the article may be monitored, and the spray pattern responsively adjusted to control the temperature of the body being sprayed. This exclusively temperature based control process, however, is only suitable when considering transformations from austenite to martensite which is only a function of temperature. It is not suited to transformations of austenite to ferrite, pearlite, or bainite because these phase changes are only partly temperature based. Because these transforms are diffusional processes that are also time-based, as well as temperature based, such transformations can occur even when temperature is held constant. Therefore merely monitoring and controlling the article's surface temperature fails to fully address the problem of internal stresses that occur during the spray forming process.
During the spray form process, the temperature of the moltenized metal droplets that are sprayed onto the ceramic model are significantly elevated above the temperature of the ceramic model and the surrounding atmosphere. Once the droplets leave the spray gun and land on the ceramic model, they become a constituent component of the article being spray formed. A portion of the heat energy input to moltenize the feed metal wire travels conductively into the ceramic model after landing, while a portion of the imposed heat remains in the body of the article being spray formed. The balance of the heat energy is dissipated out into the surrounding atmosphere which is typically the interior space of the spray form cell or enclosure in which the spraying process is taking place. As a result, different parts of the microstructure have traditionally been permitted to have different temperatures during and after the spraying process. This is especially true, for example, in the case of a large stamping tool, such as that required for stamping an automobile inner hood, if the tool were sprayed as a unibody monolith.
In another aspect, spray formed articles having complex shapes that cause different regions of the article to have relatively different locally exposed surface areas tend to cool at different rates amongst these several regions. This characteristic, in turn, affects the kinetics of the body's overall cooling profile. Different areas of an irregularly shaped article, especially an article having many undulations, tend to cool at different rates, for example, because the presence of the undulations tends to restrict heat transfer. Thus, areas within depressions of the undulations tend to be hotter than areas that protrude with a proportionately greater exposed surface area. As a result, one part of the article being sprayed can be in the bainite transformation phase, while another part is in the martensite start region.
Further, when spraying is discontinued and the sprayed article is allowed to cool to room or ambient temperature, different temperatures will begin to occur across the sprayed body. As a result, those areas loosing temperature more quickly begin to traverse the phase transformations sooner than those areas that are more heat retentive. This phenomenon is even more pronounced with a sprayed article that has a complex shape, such as those including undulations or apertures, which causes certain areas to be warmer than others until the final cooling temperature is reached and the article assumes a uniform temperature, such as equal to the temperature of the spray form cell's interior. When such articles are simply allowed to cool to room temperature in an uncontrolled manner, significant distortions are likely to occur in the article because of discontinuities across the phase transformations and stresses are created in the bodies because of these different cooling rates.
Currently available technology provides the user with an ability to monitor the exposed surface temperature of an article being spray formed. However, in spite of the recognized need, a continuing failure in the art has been a lack of means and method to accurately predict, monitor and control the more elusive, but more comprehensive, time and temperature dependent phase constituencies and volumetric changes that occur during the spray forming process. Consequently there has been a continuing inability to affect proper control over the time and temperature based phase constituencies and volumetric changes during the spray forming process for obviating the problems associated with internal stresses induced in the article being spray formed.
In view of the above described deficiencies associated with currently available spray form processes when considering time and temperature dependent phase and volumetric changes within the article being formed, the present inventions have been developed to alleviate these drawbacks and to provide further benefits to the user. These enhancements and benefits are described in greater detail hereinbelow with respect to illustrative embodiments of the inventions.